Recently, with the rapid advancement in IT (Information Technology) and the increase in humans' desire for communication, wireless communication devices, such as portable terminals and the like, have become necessities to modern people. However, the increased use of such portable terminals is making the influence of electromagnetic waves on human body into an important issue. To date, the influence of electromagnetic waves at a frequency band used by portable terminals on the human body is not yet clearly ascertained, but it has been reported that these electromagnetic waves may cause various diseases such as leukemia, brain tumor, headache, amblyopia, and when they are accumulated in the human body, brain wave disorders, men's reproductive dysfunction, and the like.
Further, there have been continuous reports of examples of undesired electromagnetic waves causing malfunctions between information communication devices. These examples are problems of EMI (Electromagnetic Interference) and EMC (Electromagnetic Compatibility) and steady research on EMI/EMC problems is under way across the world. Thus, many researches for effectively blocking electromagnetic waves to prevent the bad influences of the electromagnetic waves on the human body are being conducted.
An electromagnetic bandgap (EBG) as a technique of employing a periodic structure may be implemented by periodically arranging specifically designed unit cell patterns on a typical electric conductor at regular intervals. Since a tangential component of a magnetic field at a particular band on the surface of the EBG becomes zero, the EBG has the characteristic of preventing current from flowing through the surface. Such an EBG may be regarded as a magnetic conductor opposite to the typical electric conductor. The surface of the EBG is a high-impedance surface (HIS) in configuration of a circuit.
The frequency response characteristics of the EBG may be checked through a reflection phase which refers to a difference between the phases of an incident wave on the surface of the EBG and a reflected wave from the surface. The reflection phase of the EBG becomes zero at a resonant frequency corresponding to a high impedance surface and varies in a range from −180 degrees to 180 degrees in a frequency band around the resonant frequency. When the structural parameters of the EBG are adjusted, the reflection phase may vary.
In the structure of a typical EBG, a dielectric layer and an array layer of unit cell patterns other than a metal conductive ground plane constitute the typical structure of a frequency selective surface (FSS). FSS is a surface formed by artificially and periodically arranging specific unit cell patterns so as to selectively transmit or reflect desired frequencies. Therefore, an EBG not only completely blocks the progression of electromagnetic waves but also has the above-described unique physical characteristics, by virtue of providing a metal conductive ground plane for the characteristics of filtering of a specific frequency due to the FSS.
Conventional electromagnetic wave absorbers may be variously classified according to a type, material, absorption mechanism, and the like. To date, most electromagnetic wave absorbers have been made of materials formed to have absorption characteristics. Since such electromagnetic wave absorbers are generally developed after much trial and error, it is disadvantageous in that the manufacturing process thereof is complicated and it is highly difficult to adjust an absorption frequency band and absorption characteristics.
To address this problem, a plate-type resonant electromagnetic wave absorber such as a λ/4 wave absorber or a Salisbury screen has been proposed. Such a plate-type resonant electromagnetic wave absorber is composed of a resistive film, a dielectric spacer and a metal conductive ground plane. Therefore, it is advantageous in that, since its configuration is simplified, its manufacture can be facilitated and absorption performance can be easily adjusted, and in that, when it is constructed in multiple layers, multi-band absorption characteristics can be obtained. However, such a Salisbury screen is disadvantageous in that the thickness of the dielectric spacer from the metal conductive ground plane must be more than at least λ/4.
Therefore, an electromagnetic wave absorber, which can be easily manufactured by a simple process, can easily control an absorption frequency band and the absorption characteristics, and can adjust the thickness thereof, is required.
To implement such a required electromagnetic wave absorber, when the above-mentioned FSS is interposed between the dielectric spacer and the resistive film of a Salisbury screen, the thickness and absorption performance of the electromagnetic wave absorber should be able to be controlled owing to the inherent electromagnetic properties of the FSS.
Consequently, an electromagnetic wave absorber formed in this way has a structure formed by adding a resistive coating to the typical structure of the EBG. Furthermore, when the unit cell patterns of the EBG are designed and made of a resistive material on a metal conductor, such a resistive EBG itself may function as a simpler electromagnetic wave absorber. Such an electromagnetic wave absorber may be applied to fields where existing electromagnetic wave absorbers have been applied in order to reduce the multiple reflection of electromagnetic waves, as a simpler structure that is easily manufactured and has low cost. In particular, since the absorption frequency band of the electromagnetic wave absorber can be adjusted only by a simple structural or material deformation of the unit cell thereof, the electromagnetic wave absorber can selectively absorb the electromagnetic waves of a desired frequency band, so that this electromagnetic wave absorber can be very usefully used under the condition that electromagnetic waves of various frequency bands coexist.
However, when such an electromagnetic wave absorber is actually provided and used, since the resistive pattern layer, the dielectric layer and a metal conductor layer of the EBG are not transparent because they have inherent colors, respectively, the usage of this electromagnetic wave absorber is considerably restricted at a space where a light needs to be transmitted or at a place where a view needs to be secured. It goes without saying that a transparent electromagnetic wave absorber can be utilized in a further variety of spaces for even more different purposes. For example, when an electromagnetic wave absorber has an ability to absorb electromagnetic waves of a desired frequency band while maintaining transparency like a windowpane, it can be provided in windowpanes of facilities, such as general houses, libraries, schools, offices and hospitals, to block undesired electromagnetic waves incoming through the windowpanes, so that it can protect the human body and appliances in the facilities. However, in order to utilize a conventional transparent electromagnetic wave absorber as a windowpane, the electromagnetic wave absorber needs to be manufactured into a new windowpane due to its structure, and thus there is a disadvantage in that a previously provided windowpane cannot be utilized.